This invention relates to an improved method and apparatus for draw forming glass fibers at increased winding and production speeds.
Early glass fiber draw forming melt furnaces employed a flat nozzle plate made of platinum-rhodium alloy having a plurality of holes drilled therein approximately 1.5-3 mm in diameter. The outer bottom of such a nozzle plate has a strong tendency to become wetted by the molten glass flowing through the holes, however, and if the pitch of or spacing between the nozzle holes is made smaller to increase the hole density and production rate, and one of the running filaments is broken off, the molten glass cone formed at the nozzle exit is likely to spread across the surface of the nozzle plate. As a result, the filaments being drawn through the adjacent nozzle holes are also cut off, and this phenomenon can easily spread in a progressive manner until finally all of the filaments are cut off and the nozzle plate surface is completely covered with molten glass. If this happens it is extremely difficult to restore satisfactory forming conditions in which the molten glass filaments remain separated, and a costly shutdown and restart operation may be required.
Conical nozzle tips have been developed to overcome this difficulty, wherein the area wetted by the molten glass flowing out of a nozzle is limited to the bottom surface of the tip. The molten glass from one nozzle tip cannot "flow uphill", and thus cannot contact the molten glasses from other nozzle tips, whereby the running filaments are forced to flow separately and the nozzle spacing can be reduced to increase productivity. Limitations still exist, however, since if the tips are too close to one another the molten glass may flow into the recesses between them by capillary action, which again disrupts the forming conditions. Furthermore, it is advantageous to provide cooling fins between the tips to improve productivity, and this creates an additional tip density limitation. At the present time, for example, it is not practical for the distance between adjacent nozzle tip peripheries to be less than 4.0 mm.
The flow rate of molten glass through a passage is defined by Hagen-Poiseuille's equation as follows: EQU Q=K(.rho..pi..DELTA.P d.sup.4 /.eta.L),
where
Q: flow rate per unit time, PA1 K: proportional constant, PA1 .rho.: density of the glass, PA1 .DELTA.P: difference in pressure between the ends of the passage, PA1 d: diameter of the passage, PA1 .eta.: viscosity of the molten glass, and PA1 L: length of the passage.
As is apparent from the above equation, there are four ways to increase the flow rate of the molten glass. A first is to increase .DELTA.P by increasing the quantity of molten glass in the furnace and/or by exerting pressure on the molten glass surface. Increasing the quantity of molten glass requires a larger furnace and an attendant increase in fuel expense to maintain the melt temperature, however, and the furnace must be made air-tight before pressure can be applied to the molten glass surface which makes the construction of the apparatus unduly intricate.
A second method is to reduce the viscosity value .eta. of the molten glass by raising its temperature. This increases the fuel expense, however, and also disrupts the stability of the filament forming operation.
A third method is to decrease the value L by shortening the length of the nozzle tips, but this expedient increases the surface wetting tendency and necessitates a lower hole density to offset such tendency.
A fourth method involves increasing the diameter d of the nozzle tips, and the present invention is concerned with this expedient and with a technique for implementing a smooth and reliable filament forming start-up operation in combination therewith.
FIGS. 1 and 2 show a conventional glass fiber draw forming apparatus wherein molten glass in a melt furnace 1 flows out of nozzle tips 2 to form glass cones which in turn are drawn out into glass fibers or filaments 3. The filaments are coated with a collecting agent at a roller station 4, gathered into a single strand 6 by a roller device 5, and evenly wound on a bobbin 9 mounted on a winding stand 8 by a reciprocating guide device 7. Reference numeral 10 designates cooling fins extending between adjacent rows of nozzle tips 2, with one end of each fin being attached to a manifold pipe 11 through which a cooling agent, such as water, is pumped.
There are two methods of increasing the nozzle tip diameters, one in which the diameters are simply enlarged without changing the number of nozzle tips per unit area, i.e. hole density, and another in which the diameters are increased while maintaining the same spacing or distance between adjacent nozzle tip peripheries. In either method the molten glass flow rate per unit time, as seen from the equation, increases in proportion to the fourth power of the diameter. The molten glass cones formed at the ends of the nozzle tips thus grow quickly in size as the tip diameters are increased, and as a result the cooling effect on the cones by heat radiation to the surrounding atmosphere decreases and their temperature rises, in spite of the cooling effect provided by the fins. It thus becomes impossible to reduce the temperature of the glass from its molten level of 1300.degree.-1400.degree. C. in the furnace to a suitable filament forming level of 1100.degree.-1300.degree. C., which disrupts or disables filament formation, causes filament breakage, non-uniform filament diameters, etc. To study this phenomenon quantitatively, the distance between adjacent nozzle tips was set at 4.2 mm and the diameter of each hole was gradually increased. It was found that at molten glass flow rates of less than 0.5 g/min/nozzle, steady filament formation and continuous operation could be achieved by conventional methods. If the flow rate was increased to between 0.5 and 0.55 g/min, however, instability and non-uniformity began to occur, and above 0.55 g/min the initial filament formation and continuous operation thereafter could no longer be effected.